1. Technical Field
This invention relates to low dielectric constant, low temperature fired glass ceramics especially useful in the manufacture of multilayer integrated circuits, thick film hybrid circuits and other electronic components, and comprising a glass from the CaO-B.sub.2 O.sub.3 -SiO.sub.2 glass system.
2. Description of the related art
Cofired multilayer circuits (CMC) are known in the art. Advances in information processing technology have resulted in ever increasing numbers of circuit elements per unit area. A concurrent need for CMC substrates satisfying certain performance criteria has arisen. The desirable characteristics that a CMC substrate should exhibit are a low dielectric constant, a thermal expansion coefficient (CTE) matching that of the chip, high thermal conductivity and amenability to be processed into cofired multilayer circuits at low temperatures.
Multilayer circuits can be prepared from endless tapes containing dielectric material dispersed in a binder-solvent solution and cast onto an endless flexible film by techniques known in the art. Typical dielectric materials have included Al.sub.2 O.sub.3 as a refractory filler or crystalline phase in a non-crystalline glass (glass/ceramic composite systems).
Composite ceramics incorporating alumina have been used as substrates for a long time. Current conductor, resistor and dielectric systems have properties, such as the coefficient of thermal expansion, matched to that of alumina. Early ceramics employing alumina required temperatures of 1400.degree.-1500.degree. C. for sintering. Such high temperatures presented problems well known in the art, such as limiting conductor metallurgy to refractory metals such as molybdenum and tungsten, the use of which presented the further problem of oxidizing at sintering temperatures. The oxidation of the conductor metals necessitated firing in a neutral H.sub.2 atmosphere.
These difficulties lead to attempts at developing CMC substrate materials that could be fired at temperatures below about 950.degree. C. where silver and copper can be used.
Glass ceramic composites were considered good candidates for CMC substrate materials because of low dielectric constants and low sintering temperatures. Glass ceramic composites, however, suffer the drawback of exhibiting low thermal conductivities. The current art, then, is comprised of composite materials that are the result of judicious compromises among the various requirements.
The speed of signal transmission in hybrid microelectronic circuits is affected by the dielectric constant of the insulating layer. Attempts at fabricating lower dielectric materials for use in thick film hybrid microcircuits have included polymeric dielectric materials, porous dielectric materials, and glass-ceramic composite dielectric materials.
The glass-ceramic composite materials have also been good candidates for thick film substrates because of their superior mechanical, electrical, and hermetic properties compared to porous dielectrics. They also have superior mechanical, physico-chemical, and refractory properties compared to polymeric dielectrics. The desirable characteristics that a glass-ceramic for use in thick film pastes should exhibit are a low dielectric constant, a thermal expansion coefficient (CTE) compatible with that of the substrate, excellent hermetic and insulating properties, compatibility with precious metal pastes, and amenability to be processed into thick film hybrid circuits at temperatures below about 950.degree. C.
Thick film hybrid circuits can be prepared from inorganic powders made up into pastes with an organic vehicle; the pastes are applied to a suitable substrate, such as alumina, and then fired. Typical dielectric materials have included Al.sub.2 O.sub.3 as a refractory filler or crystalline phase in a non-crystalline glass (non-crystallizing glass/ceramic composites).
U.S. Pat. No. 4,654,095 teaches such a dielectric composition for use in tapes in the fabrication of multilayer circuits wherein selected non-crystallizing glass having certain deformation and softening temperatures are mixed with a refractory filler that is insoluble in the glass, and then fired at 825.degree. C.-900.degree. C.
U.S. Pat. No. 4,642,148 describes a multilayer system requiring a two-stage firing in which the substrate is composed of alumina, a crystallizable glass such as crystallizing spodumene or cordierite, and a non-crystallizing soft glass such as borosilicate or aluminosilicate glass.
U.S. Pat. No. 4,621,066 teaches low temperature fired (800.degree. C. to 1000.degree. C.) composite ceramics for use in electronic components wherein non-crystalline glass from the CaO-SiO.sub.2 -Al.sub.2 O.sub.3 system is mixed with 35 to 50% alumina.
U.S. Pat. No. 4,598,167 teaches composite ceramic substrate materials that are mixtures of a low temperature softening glass and various forms of SiO.sub.2, and sinterable at temperatures below about 900.degree. C. The dielectric constants are reported to be in the range of 4-6, and the CTE in the range of 4.5 to 7.0 ppm/.degree. C.
U.S. Pat. Nos. 4,413,061 and 4,301,324 describe two glass ceramic systems having spodumene and cordierite as the main crystallizing phases. The dielectric constants are reported as in the range of 5.0 to 6.5, and the CTE as in the range of 2.0 to 8.3 ppm/.degree. C.
U.S. Pat. No. 3,816,172 teaches a partially crystallized dielectric comprising glass crystals in a glassy matrix.
U.S. Pat. No. 4,714,687 teaches a low k glass ceramic suitable for dielectric substrates and comprising willemite and cordierite.